Magnetic stirrers with a rotatable magnet driven by a drive motor are known. Upon activation, this rotatable magnet is magnetically coupled with a stirring magnet. This stirring magnet is placed within a container with the medium that is to be stirred and is, as a rule, located above the rotatable magnet.
In conjunction with such stirrers it is also known that during the stirring procedure the viscosity of the medium or fluid can increase or decrease. Often, such changes in viscosity are actually so slight that they cannot be detected at all or are detected only very inexactly through use of conventional means and methods. Up until now, for example, a test sample is taken from the stirring container, its viscosity measured, and the entire sample then poured back into the container so that the total quantity of stirred fluid in the stirring container is not altered. During the removal and transfer of the test sample to a measurement site, however, additional changes in viscosity can also take place, so that an inaccurate measurement may result. In addition, the removal itself of the test sample and its pouring back can in turn lead to slight changes in viscosity.
It is, of course, already known that the current consumption of drive motors can be measured, and thus a change in the power consumption can be detected in the event of load changes. However, this is only successful when relatively large changes in the load take place. Since, however, in the case of viscosity variations and, above all, small changes in viscosity of the liquid to be stirred, the load changes on such a drive motor for a magnetic stirrer can be much smaller than, for example, the network variations of the power supply, a reliable value for the tendency of a change in viscosity cannot be attained in this way.
From DE-AS 1 804 292 an apparatus is known for the determination of the prothrombin time of blood plasma, an apparatus that is based on the fact that during the coagulation of the blood plasma, a stirring magnet is prevented from further rotation as a result of the increasing viscosity, so that with the aid of an inductive circuit element, an electrical signal can be generated in order to indicate this point in time. In this way, however, only the coagulation of the blood plasma can be indicated, and not a very slight change in the viscosity. Above all, the tendency of a change in viscosity cannot be determined in this way.
From DE 23 66 207 C2 an apparatus is known for the measurement of cure times, particularly for plastic resins of different generic types, in which a stirring head is placed at a location provided with a rotating drive. A disconnect device for the rotating drive as well as a timer, is provided that switches when there is a sharp increase in the stirring resistance. In conjunction with this, a disconnect apparatus is also provided that activates at a pre-determined limit value during the increase in current consumption of the rotating drive resulting from the increase in the stirring resistance. The tendency of a change in viscosity cannot be reliably determined in this way either.